Continuous manufacture of chlorates and perchlorates



June 30, 1970 CRANE 3,518,173

CONTINUOUS MANUFACTURE OF CHLORATES AND PERCHLORATES Filed Dec. 26. 196'? United States Patent 3,518,173 CONTINUOUS MANUFACTURE OF CHLORATES AND PERCHLORATES George J. Crane, Islington, Ontario, Canada; Ernest Arthur Du Vernet and Selma Leslie Crane, executors of the last will of said George J. Crane, deceased, assignor to Huron Nassau Ltd., Nassau, Bahama Islands, a corporation of the Bahamas Continuation-impart of application Ser. No. 362,720, Apr. 27, 1964. This application Dec. 26, 1967, Ser. No. 693,551

Int. Cl. C01b 11/26 US. Cl. 204-95 4 Claims ABSTRACT OF THE DISCLOSURE The operation of a plurality of multipolar electrolytic cells in the production of an alkali metal chlorate or perchlorate, particularly sodium chlorate or perchlorate, by the electrolysis of an electrolyte comprising an aqueous solution of an alkali metal chloride on a continuous basis. In the continuous operation, the electrolyte is partially electrolysed in a first cell and further electrolysed in subsequent cells, in each cell the electrolyte being continuously recirculated through the cell from a cell tank to effect electrolysis, a small proportion of the recirculating volume of the electrolyte in each cell being continuously passed serially between the cells, fresh electrolyte being continuously fed to the first cell and electrolysed electrolyte being continuously withdrawn from the last cell of the series. By the improved operation of the multipolar cell according to the present invention, the overall efiiciency of the process for the production of the alkali metal chlorate or perchlorate is substantially increased.

This application is a continuation-inpart of US. application No. 362,720 filed Apr. 27, 1964, now abandoned.

The present invention relates to the production of alkali metal chlorates and perchlorates by the electrolysis of an electrolyte comprising an aqueous solution containing an alkali metal chloride in multipolar electrolytic cells. In particular the present invention relates to the production of such alkali metal chlorates and perchlorates in which the multipolar cells are operated upon a continuous basis, and in which the electrolyte is partially electrolysed in each of a series of cells, a small portion of the electrolysed electrolyte from each cell being serially continuously passed to the next adjacent cell for further electrolysis.

The electrolytic production of an alkali metal chlorate or perchlorate particularly sodium chlorate or perchlorate has heretofore been effected by the electrolysis of an electrolyte comprising an aqueous solution containing an alkali metal chloride suitably at a pH in the range 6-7 and more preferably in the range 6.56.8, the electrolyte usually containing electrolytic aids such as acid chromate. While initially such electrolysis was conducted in monopolar electrolytic cells, it has been found that the electrolysis of the electrolyte in multipolar electrolytic cells has many advantages over operation in monopolar electrolytic cells. However, while it is conventi nal to operate monopolar electrolytic cells on a continuous basis effecting partial electrolysis of the electrolyte during a single pass of electrolyte in each cell, due inter alia to volume and electric power consideration it has heretofore been considered commercially necessary to operate the electrolysis in multipolar electrolytic cells on a batch basis even allowing for the recognized disadvantages of batch operations such as loss of time in stopping and starting the process including emptying and filling the Patented June 30, 1970 ice cell tanks in which the multipolar cells are normally located.

A multipolar electrolytic cell with which the present invention is concerned, is an electrolytic cell which consists of a number of cell units which are electrically connected in series but through which the electrolyte flows in parallel. It is formed from a pair of monopolar electrodes which have the same polarity on all surfaces and through which surfaces current enters or leaves the cell at the same polarity relative to the surrounding electrolyte. Dividing the space between the monopolar electrodes are a plurality of spaced intermediate bipolar electrodes which are at a positive polarity on one side and a negative polarity on the other side with respect to the electrolyte. In such a cell the total voltage impressed between the two monopolar electrodes which is typically of the order of to 250 volts is taken up by the bipolar electrodes such that the sum of the difference between the voltage on adjacent bi-polar electrodes is equal to the aforesaid total voltage. Thus, for instance, in a cell containing 24 bipolar electrodes operating at a voltage of 100 volts between the monopolar electrodes there would be a difference of 4 volts across each pair of adjacent electrodes (which form a cell unit) and there would be 25 individual separate electrolytic cell units between each pair of monopolar electrodes each of which units operates at a different voltage. The multipolar electrolytic cell in the production of the alkali metal chlorates and perchlorates is normally located in a tank which contains the electrolyte, the electrolyte being recirculated in parallel through the cell units conventionally from the bottom of the cell to the top of the cell and back into the tank during which pass through the cell it is subjected to electrolytic action i.e. electrolysis. During its residence time in the tank the relatively slow chlorate producing reaction involving the combination of the hypochlorous acid and the hypochlorite ion according to the equation 2HClO+OCl'=ClO +2HCl takes place, the hypochlorous acid and hypochlorite ions being generated by a relatively fast reaction occurring in the electrolytic cell. In conventional batch operation of the multipolar electrolytic cell system for the production of sodium chlorate and perchlorate, the recirculation of the electrolyte through the cell from the tank is continued until the desired concentration of sodium chlorate or perchlorate is obtained in the electrolyte in the tank whence the cell operation is then shut down, the electrolyte removed from the tank and sent for further processing for the recovery of the sodium chlorate and perchlorate, the cell tank and the cell cleaned and then filled up with fresh brine solution for subsequent batch operation. 'llhe completion of the electrolysis process normally takes from 5 days to 2 weeks depending upon the liquid capacity of the tank in relation to the electric power applied to the multipolar cell. During the process, the electrolyte is enriched periodically with more brine when a chlorate is manufactured or with a chlorate solution when a perchlorate is manufactured. In addition, water is periodically added to replace that which has been electrolysed or evaporated during the process. In: a typical chlorate plant a number of multipolar electrolytic cells and their tanks will be present all of which are operated upon a batch basis and the usual output from a single tank varies from 200-2000 tons per year depending upon its size.

In contrast to the multipolar electrolytic cell with which the present invention is concerned, in a monopolar cell each cell is an integral unit having monopolar electrodes only and in each cell unit there is a separate flow in and out of the electrolyte and a separate flow of electricity from and to each cell unit.

In monopolar cell operation with all the cells in series the total volume of electrolyte is typically of the order of 2000 cubic feet and from this point of view, it would be extremely difficult to operate monopolar cells on a batch basis as in a batch system using for instance 40 cells, there would have to be 40 separate operations and 40 storage tanks and it has become in the operation of monopolar cells standard practice to operate monopolar cells with the electrolyte flow in series, the electrolyte passing between the electrodes of each cell once only. In contrast to the 2000 cubic foot flow of electrolyte in the monopolar cell system, in a multipolar cell system having say 4 cells the electrolyte volume would be something in the order of 20,000 cubic feet and in view of the relatively large volume involved and inherently in a multipolar electrolytic cell, you have a large tank for each cell each with a volume of electrolyte greater than the total of the monopolar cells, it has not heretofore been considered necessary or even desirable to operate these multipolar electrolytic cells in series and it has been considered conventional to operate multipolar electrolytic cells in the production of alkali metal chlorates and perchlorates on a batch basis of about 8-14 days per batch per cell.

In similar vein, operation of 40 monopolar cells in series which typically operate at 50 kw. involves 2,000 kw. in the whole system. However, with a single multipolar electrolytic cell operation there is somewhere between 1,200-2,500 kw. loading and it will be readily seen that from the electrical point of view, a single multiplar cell operating on a batch basis is equivalent to a plurality of monopolar cells operating upon a series basis.

Thus, heretofore, it has not been considered to be worthwhile to operate multipolar electrolytic cells in the production of an alkali metal chlorate or perchlorate on a continuous basis because a single multipolar electrolytic cell operating on a batch basis is involved with a volume of electrolyte substantially in excess of the volume of electrolyte which can be handled by a plurality of monopolar electrolytic cells operating in series, and further a single multipolar cell operating on a batch basis involves the same electrical loading as a plurality of monopolar electrolytic cells operating in series. In particular, it has heretofore been considered that a multipolar electrolytic cell operating on a batch basis is equivalent to a plurality of monopolar electrolytic cells operatigng upon a series basis and there is no equivalency whatsoever between operating a monopolar cell system on a continuous basis and operating a multipolar cell system on a continuous basis as completely different considerations are involved. However, while as aforesaid operation of a multipolar cell on a continuous basis has not heretofore been considered to be economically advantageous, it has been considered that there are decided disadvan tages to operating a multipolar electrolytic cell in the production of an alkali metal perchlorate or chlorate on a continuous basis and up to the present invention all major sodium chlorate and perchlorate producers in the United States and Canada who were producing sodium chlorate and sodium perchlorate using monopolar cells and multipolar cells have been running their monopolar cells on a series basis and their multipolar cells on a batch principle. In particular, it was theretofore considered to be disadvantageous to operate multipolar electrolytic cells in the production of sodium chlorate or perchlorate on a continuous basis as after every batch operation in the multipolar electrolytic cell operation the cell had to be pumped clear and washed out thoroughly to remove the accumulation of graphite dust and crud and it was thus considered in view of the fact that this cleaning was necessary there would be no advantage in operating continuously, because each cell would have to be periodically emptied and cleaned out anyway, so batch operation of the multipolar electrolytic cells seemed the natural way to effect the operation. Thus, to operate on a continuous basis, it would be necessary to provide each multipolar cell with some means of removing the graphite dust and crud during the operation of the cell which would entail capital expenditure in research which was not heretofore considered to be worthwhile.

Further, operating the multipolar electrolytic cell on a batch basis, the electrolyte starts out with a pure brine and finishes up with an electrolyte containing a mixture of sodium chloride and sodium chlorate. The electric power requirement is a function of the voltage across the cell and the voltage is a function of the sodium chloride concentration and hence batchwise operation it was considered would give greatest efficiency because with series electrolytic flow the average sodium chloride concentration is lower than that with batch operation.

In summary therefore, as the state of the art stood before the present invention, it was generally considered that operation on a continuous basis of multipolar electrolytic cell in the production of sodium chlorate and perchlorate was not necessary and there were obvious, on the face of it, disadvantages in effecting such a process and the teaching of the prior art is against using a continuous operation.

It has now been found according to the present invention that there are advantages to operating a multipolar electrolytic cell on a continuous basis which far outweigh the aforesaid disadvantages set forth heretofore which advantages were not obvious before the present invention. Further, applicants have found that the optimum advantages are to be obtained by operating the multipolar cells on a continuous basis and in particular effecting partial electrolysis of the brine in each of the multipolar electrolytic cells and continuously passing a small proportion, usually of the order of 10%, of the recirculating volume of electrolyte in each of the multipolar electrolytic cells to a subsequent cell for further electrolysis, fresh electrolyte being added to the first of the electrolytic cells and electrolysed electrolyte being removed from the last of the electrolytic cells.

According to the present invention therefore, there is provided in the production of a compound selected from an alkali metal chlorate and an alkali metal perchlorate particularly sodium chlorate and sodium perchlorate which includes electrolysing an electrolyte comprising an aqueous solution of an alkali metal chloride in a plurality of multipolar electrolytic cells in which the electrolyte is continuously recirculated through each cell during electrolysis from an individual cell tank the improvement which comprises providing a continuous serial flow of a small proportion of the volume of electrolyte recirculating in each cell tank between said cells, partially electrolysing the electrolyte in the first of said cells and effecting further partial electrolysis of said electrolyte in each of the subsequent cells of said series, continuously introducing fresh electrolyte into the cell tank of said first cell and continuously removing electrolysed electrolyte from the cell tank of the last of said cells of said lsjeries whereby to effect the process on a continuous asis.

Thus, applicants have found in contrast to conventiona1 thinking in the operation of multipolar cells for the production of sodium chlorate and sodium perchlorate it has now been unexpectedly found that continuous opera tion according to the present invention gives greater efficiency than batch operation because it is possible by continuous operation to control and maintain constant the temperature of the electrolyte, its pH and the hypochlorite content of the electrolyte in each stage of the process, i.e. in each multipolar electrolytic cell system.

In particular, with batch operation initially the temperature of the brine is at ambient temperautre, for instance 20 C. and at 20 C. to operate the process more power, i.e. kilowatts are required than operating at a temperature of 40 C. Therefore, when operating on a batch basis until the temperature of the electrolyte reaches 40 C. the efliciency of the process is not optimum. On the other hand, operating on a continuous basis as according to the present invention makes is possible to maintain the temperature of the liquor in all the tanks at optimum operating temperature, e.g. 40 C. and therefore the process is always operating at this optimum temperature and this increases the efficiency of the process. Further in the proc ess starting with fresh brine, the pH changes extremely rapidly because the sodium chloride must first break down into sodium hypochlorite and hypochlorous acid before forming the sodium chlorate. The product is thus initially a strong base and a weak acid and in practical terms when fresh brine is put into the electrolytic cell with an adjusted pH of 6.8, as is required in the process, after 20 minutes operation ,the pH would be about 8 and more acid must be added quickly to bring the pH close to 6.8 so that the electrolyte contains the correct proportions of hypochlorous acid and sodium hypochlorite. If the proportions are incorrect, power consumption increases as much as 50% because the sodium chlorate is made by different chemical route. In contarst thereto, operating on a continuous basis it is possible to automatically maintain the pH of the electrolyte and the hypochlorite content of the electrolyte essentially constant and thus avoid excessive power consumption.

Again, operation of the multipolar electrolytic cell according to the present invention and the production of sodium chlorate and perchlorate on a continuous basis has the advantage of reducing the loss of time required to empty and replenish the multipolar cells operating on a batch basis and further avoid the use of large storage containers which are necessary in a batch process and equalizes out the voltage considerations which are required due to the changing composition of the electrolyte on a batch basis. In particular, during theemptying of a cell tank after completion of a batch process, the entire facility on the same circuit must be disconnected and since emptying and filling the tank requires approximately 1 to 3 hours a typical plant of 4 to 8 cell tanks on one circuit will incur a lost production time of 5-15 Further, operating the electrolysis in the multipolar electrolytic cells on a batch basis means that the electrolyte in each of the cell tanks must be separately analyzed and controlled by adding the various materials such as water, brine, acid chromate, and soluble calcium salts when they become depleted in the electrolyte and further as the electrolyte becomes enriched in the sodium chlorate the voltage necessary for maintaining the current flow through the cell is increased. As an example a freshly filled tank would require only 100 volts for a full production rate but the same tank when electrolysis is finished would require about 120 volts. Since this variation takes place across each cell tank the voltage supply must be variable over the relatively large range. This increases the capital cost of equipment and increases the peak amount for electrical energy and hence the cost per kw. hr. of electric power will increase. By operating on a continuous basis it is possible to operate each cell at a substantially constant voltage depending upon the concentration of sodium chloride in each cell tank and thus the demand for electrical energy will remain substantially constant there being no peak demand.

Again according to the present invention when operating a multipolar electrolytic cell on a batch basis the electrolyte will remain in each cell for something of the order of 5 days to 2 weeks whereas with the process of the present invention the electrolyte remains in each cell tank for only a few hours and further the accurate control of each cell tank as is possible according to the present invention decreases the operating hazard through reduction of foam containing hydrogen or oxygen. Still further, by operating on a continuous basis according to the present invention should a cell operation fail, or otherwise become unmanageable it can be taken out of operation and bypassed with the electrolysed effluent from the preceding cell without necessitating an overall shutdown.

Finally a substantial advantage according to the invention is whereas when the cells are operated on a batch basis, they require a wash-down and particular removal procedure every week or two, when operating according to the present invention this problem is removed and the graphite and crud level in the cells are no substantial problem. Thus, due to the continuous removal of the product from the top level of the electrolyte the harmful particles are continuously removed as formed and are pumped out to the product collector tank. They do not have time to settle or coagulate and no down time is required for washing. Further operation is consistently better because the cells do not clog with extraneous material which is unexpected and effectively increases the annual cell output capacity by some 2% because of shutdown elimination.

The present invention will be further illustrated by way of the accompanying drawing which is a perspective side elevation of three multipolar electrolytic cells and their associated tank the third of which is partially broken away adapted for operation according to the present invention.

Referring to the drawing there is provided a third multipolar electrolytic cell 1 disposed in a tank 2 on a block 3, the tank 2 containing the electrolyte surrounding the cell wall, the electrolyte being cooled by means of coils 4 disposed therein through which coils 4 water is passed via lines 5 and 6. The electrolyte is passed into the electrolytic cell 1 through inlets 7 in a block 8 attached to the outside walls of the cell 1 and pass upwardly through the cell 1 either by natural circulation or by means of a pump (not shown) and back into the cell tank 2 via the outlets 9 in the block 10 also attached to the outside of the walls in the cell 1.

Located in the cell 1 are a pair of monopolar electrodes 11 one of which is connected to the positive of an electrical supply by means of a copper conductor 12 and the other of which is connected to another monopolar electrode 11a in the second cell 1a by means of the c0pper conductor 13. Between the monopolar electrodes 11 in the cell 1 are bipolar electrodes 14 which effectively divide the cell up into a series of cell units each of which has an inlet 7 and an outlet 9 in communication therewith such that the electrolyte recirculating through the cell 1 passes in parallel throught the cell units formed between the electrodes 14 and also between the adjacent electrodes 14 and 11. During passage through the cell 1 the electrolyte which comprises an aqueous solution containing sodium chloride and other electrolytic aids at a pH of approximately 6.8, is partially electrolysed to produce hypochlorous acid and sodium hypochlorite which then converts slowly to sodium chlorate in the cell tank 2.

Disposed beneath the outlet 9 in the block 10 is a trough 15 which collects the efiluent from the outlet 9 before passage to the tank 2 by means of overflowing from the trough 15 and a small proportion of the electrolyte is continuously removed from the tank 2 according to the present invention by means of the scoop 16 at the end of the arm 17 which is adapted to rotate on the axis 18 in the direction shown by the arrows. The electrolyte on rotation of the arm 17 is passed into the channel member 19 from whence it is taken for further processing for the recovery of the sodium chlorate from the liquor. Electrolyte passes into the cell tank 2 from the cell tank 2a by means of channel wires 19a bridging the cell tanks 2 and 2a and supplied from a trough 15a in the cell tank 2:: by means of scoops 16a which remove electrolyte from the trough 15a on rotation of the arm 17a on the axis 18a in a similar manner as aforesaid.

The second multipolar cell 1a and its associated tank 2a are of the same structure as the multipolar cell 1 and its associated tank 2 as is the first multipolar cell 1b and its associated tank 2b, the only difference being that the 7 monopolar electrode 11b in the first cell 11) is connected to a negative source of power through the copper conditions of operation of each of the five cells is given in the following table:

TABLE Feature No. 1 Cell No. 2 Cell No.3 Cell No. 4 Cell No. 5 Cell HClO and NaClO in electrolyte (g.p.l.) 3 3 3 3. 2 3. 3 NaCl, g.p.l. in electrolyte 288. 6 265. 6 240. 7 213.6 185 NaClO g.p.l. in electrolyte 56. l 115 177. 5 243. 5 312 Voltage to ground 2l0 l06 +110 +222 CaClz, g.p.l. in electrolyte. 2 2 2 2 .2 Sodium diehrornate in electrolyte 0. 2 0. 2 0. 2 0. 2 0. 2 Flow, U.S. g.p.n1 13. 2 to 13. 2 to 12. 9 to 12. to 12. 2

ductor and the cell tank 2b is applied with a fresh aqueous solution of sodium chloride by means of line 21 and a pump (not shown).

In operation the electrolyte in each cell recirculates around the cooling coils 4 and through the cell between the electrodes where it undergoes partial electrolysis. The rotating scoops 16, 16a and 16b continuously transfer electrolyte from the trough .15, 15a and 15b into the channel members 19, 19a and 19b whereby a small proportion usually in the order of 10% is transferred continuously to the next cell tank, fresh brine in an equal volume being introduced through line 21 into the first tank 2b. In a typical cell with 10.0 bipolar cell units operating at 10,000 amps the recirculation rate of the electrolyte in any particular cell tank will be of the order of 500-800 gallons per minute and there will be a forward transfer flow from cell to cell of something of the order of 10 gallons per minute. The drawings of the present invention are of course simplified drawings for the purposes of understanding the present invention and thus there are only three cell units so shown with their associated cell tanks and the number of electrodes in the cells of course does not represent the conventional actual number which as aforesaid is of something of the order of 100 electrodes or more.

The present invention will be further illustrated by way of the following example.

EXAMPLE Five multipolar electrolytic cells were set up for serial flow of electrolyte substantially as shown in the accompanying drawing, each cell containing 120 graphite electrodes, five of which were monopolar electrodes and the rest, bipolar electrodes. The volume of electrolyte recirculating in each cell was approximately 20,000 cubic feet the temperature of the recirculating electrolyte was maintained at about 40 C. and the pH of the electrolyte was maintained at 6.8 by the periodic automatic injection of the hydrochloric acid therein, the ratio of hypochlorous acid to sodium hypochlorite in the recirculating electrolyte in each cell being maintained automatically by maintenance of said pH at about 1 to 2. Brine containing 310 grams per litre of sodium chloride a trace of calcium chloride and a small amount, 0.2+ or 0.1 gram per litre, of sodium dichromate was fed at a rate of 13.5 US. gallons per minute to the first cell. The relevant con- From the last cell numbered 5 a liquor was obtained at a rate of 11.8 US. gallons per minute containing 1.85 grams per litre of sodium chloride, 312 grams per litre of sodium chlorate, 0.2 gram per litre of calcium chloride and 0.2 gram per litre of sodium dichromate which was then for further processing. The cells were all operated at a current of 900 amperes and the voltage to ground was periodically reversed to even out anode and cathode wear.

I claim:

1. In the production of a compound selected from an alkali metal chlorate and an alkali metal perchlorate which includes electrolysing an electrolyte comprising an aqueous solution of an alkali metal chloride in a plurality of multipolar cells in which the electrolyte is continuously recirculated through each cell during electrolysis from an individual cell tank the improvement which comprises providing a continuous serial flow of a small proportion of the volume of electrolyte recirculating in each cell tank between said cells partially electrolysing the electrolyte in the first of said cells and effecting further partial electrolysis of said electrolyte in each of the subsequent cells of said series continuously introducing fresh electrolyte into the cell tank of the first cell and continuously removing electrolysed electrolyte from the cell tank of the last of said cells of said series whereby to effect the process on a continuous basis.

2. The process as claimed in claim 1 in which the alkali metal is sodium.

3. The process as claimed in claim 1 in which up to 10% of the recirculating electrolyte in a cell tank by volume is continuously removed and passed into the cell.

4. The process as claimed in claim 1 in which the small proportion of electrolyte is removed from the surface of the electrolyte in the cell tank.

References Cited UNITED STATES PATENTS 3,287,251 11/1966 Horne et al 204 DANIEL E. WYMAN, Primary Examiner I. M. HICKEY, Assistant Examiner US. Cl. X.R. 20482 

